1. Field of the Invention
The present invention relates to formation of a metal thin film and, more particularly, to a method of controlling film formation conditions of a thin film required to obtain predetermined film quality.
2. Description of the Related Art
Sputtering, heating deposition, ion plating, plating, and the like are known as a conventional method of forming a metal thin film. In any of these methods, it is important to reduce an impurity amount mixed during a formation process in order to obtain good film quality. The following two methods have been typically, conventionally adopted as a method of reducing the impurity amount to obtain a high-purity metal thin film.
In the first place, the purity of a thin film raw material is improved. Since purity management of a raw material can be comparatively easily performed in mass-production techniques, it is generally performed.
In the second place, an atmosphere in a reaction or formation chamber is kept at a predetermined level to reduce contamination, thereby preventing mixing of an impurity during a thin film formation process. It is difficult to perform an in-process measurement of mixing of an impurity during the film formation process, and this measurement is often unsuitable especially for a mass-production process. In order to control the atmosphere in a reaction or formation chamber, analysis of residual impurity components in the chamber is used. Alternatively, the concentration of an impurity mixed in a thin film is estimated on the basis of characteristic values of a completed thin film, e.g., a hardness, a grain size, and a specific resistance of the thin film, and this information is fed back to the formation process to control the atmosphere in a chamber.
When an Al thin film is to be formed by a sputtering method, for example, film quality depends on a residual gas in a sputtering chamber. Although a mass spectrometer is used to measure this residual gas, it is practically impossible in a mass-production process to continuously measure impurities such as H.sub.2 O, N.sub.2, O.sub.2, and CO mixed on the order of several tens ppm in Ar gas used in the sputtering step.
For the above reasons, physical characteristics of a formed film are measured, and normality of formation conditions is determined in accordance with the measurement values. In a mass-production process, information indicating this determination must be rapidly fed back to a formation step. The normality of thin film formation conditions is empirically determined in accordance with a Knoop hardness H.sub.K or a specific resistance .rho. of a thin film. Since, however, the physical characteristic values such as a hardness, a grain size, and a specific resistance described above are insensitive to a change of a thin film microstructure, i.e., an impurity amount precipitated in a grain boundary, they cannot be correct indexes for controlling the formation conditions.